1. Field of the Invention
The present invention relates to an organic electroluminescent (EL) device, and more particularly, to an organic EL device based on top-emission and a method for fabricating the same.
2. Discussion of the Related Art
Recently, studies or organic matters such as a conjugate polymer having conductivity have been actively performed since development of an organic EL device based on poly-phenylenevinylene (PPV) that is one of the conjugate polymer. Studies for application of such organic matters to a thin film transistor (TFT), sensors, lasers, photoelectric device, and so on have been actively performed. Of them, studies of the organic EL device have been performed most actively. An inorganic EL device of inorganic matters based on phosphors requires an operational voltage of 200V or greater as an alternating current voltage and is fabricated by vacuum deposition. For this reason, the inorganic EL device has drawbacks. It is difficult to obtain a large-scaled inorganic EL device and particularly emit blue light. Also, the fabricating cost of the inorganic EL device is high.
However, the organic EL device of organic matters has advantages in that it has excellent light-emitting efficiency, facilitates a large-scaled area, and simplifies the process. Particularly, in the organic EL device of organic matters, it is possible to easily emit blue light and obtain an EL device that can be curved. In these respects, the organic EL device has received much attention as a next generation display device. Particularly, an active matrix EL device having an active driving device in each pixel is being actively studied as a flat panel display device in the same manner as a liquid crystal display (LCD) device.
FIG. 1 illustrates a related art active matrix organic EL device. Bottom emission is shown in FIG. 1, in which light is emitted toward a substrate. As shown in FIG. 1, a TFT 60 is formed on a substrate 10 of transparent glass, and an anode 20 is formed to overlap source and drain electrodes of the TFT 60. Subsequently, an insulating film 30 is formed on the TFT 60, and an organic EL layer 40 and a cathode 50 are sequentially formed on the entire surface of the insulating film 30.
In the aforementioned active matrix EL device based on bottom emission, a light-emitting portion is covered with the TFT 60 formed between the substrate 10 and the organic EL layer 40, when light emitted from the organic EL layer 40 is emitted through the substrate 10. In this case, an aperture ratio of the active matrix organic EL device is geometrically reduced if the size of the TFT 60 or the number of the TFTs 60 is increased. As a result, it is difficult to use the active matrix EL device as the display device. Therefore, to prevent the aperture ratio from, being reduced due to the TFT 60, top emission has been employed, in which light is emitted to an opposite surface of the substrate.
FIG. 2 illustrates a related art active matrix organic EL device based on top emission. As shown in FIG. 2, a TFT 60 is formed on a substrate 10, and a via hole is formed to expose source and drain electrodes of the TFT 60. A planarization film 80 is then formed on the entire surface of the substrate 10. An anode 20 is formed on the planarization film 80 to contact the source and drain electrodes of the TFT by way of the via hole. Subsequently, for electrical insulation, an insulating film 30 is formed in a contact region between the source and drain electrodes of the TFT 60 and the anode 20. An organic EL layer 40 and a cathode 50 are sequentially formed on the entire surface of the insulating film 30.
In the active matrix organic EL device based on top emission, the anode 20 serves as a reflecting surface to reflect light toward the cathode 50, and the reflected light is emitted to the outside through the cathode 50 that is a transparent electrode. However, since the anode is planarized in the organic EL device based on top emission, light reflected from the surface of the anode is scattered in all directions.
FIG. 3 is an enlarged view illustrating a portion A of FIG. 2 in detail. As shown in FIG. 3, since the surface of the anode 20 formed on the planarization film 80 is planarized, light reflected from the surface of the anode 20 is scattered in all directions. In this case, since an amount of light toward the cathode is small, light efficiency of the organic EL device is deteriorated.